Is the Ending Slash Optional?

HTML4: the slash is technically invalid. However, it's accepted by W3C's HTML validator.

XHTML: The slash is REQUIRED.

I recommend to always add the slash. Because, it provides a visual clue of non-closing tags.

XHTML Supports Self-Closing Syntax for All Tags

In XML/XHTML, any tag can be self-closed by the ending slash <… />.

When you test XML, you have to make sure the server is sending the correct Internet media type as XHTML (For example, application/xhtml+xml), otherwise browsers will treat it as HTML, regardless of the DOCTYPE in your file.